This invention relates to a sheet feeding apparatus for feeding originals, copy sheets or other documents to a copying machine or to a facsimile apparatus, and more particularly to an air suction type sheet feeding apparatus in which sheets are individually sucked upwards by a negative pressure from a top sheet of a sheet stack placed on a sheet receptacle and under the condition, a sheet feed roller is rotated so that the sucked sheets are individually fed into the copying machine or facsimile apparatus.
The air suction type sheet feeding apparatus has an excellent feature capable of feeding sheets individually with different sizes and rigidities since sheets are individually sucked from a top sheet of the sheets stacked on a sheet receptacle. Therefore, practically it is mainly used as the sheet feeding apparatus of facsimile apparatus.
As a sheet feeding apparatus of the air suction type, an apparatus as shown in FIG. 1 is known. This apparatus comprises a sheet receptacle 2 for placing a stack of sheets 1 thereon, a vacuum plate 3 which is disposed above the sheet receptacle 2 and which has a sheet suction hole 3a in the bottom thereof directed to the sheet receptacle 2, a casing 4 which covers the vacuum plate 3, a fan 6 for sucking air from the casing 4 in order to bring a space 5 between the vacuum plate 3 and the sheet receptacle 2 to a negative pressure state, a sheet feed roller 7, which is disposed rotatably in the casing 4 and a sheet transport portion thereof is exposed to the side of the sheet receptacle 2 through the vacuum plate 3, a sheet separating roller 8 which is disposed rotatably downstream of the sheet transport path with respect to the sheet feed roller 7, namely on the right in FIG. 1, a frictional sheet separating plate 9 which is disposed in light contact with the peripheral surface of the sheet separating roller 8, and a drive apparatus 10 for rotating each of the above-mentioned rollers in the sheet transporting direction.
In this sheet feeding apparatus of FIG. 1, the fan 6 is rotated so that the atmospheric pressure in the casing 4 which covers the sheet feed roller 7 is made negative, whereby the space 5 between the vacuum plate 3, which constitutes a bottom plate of the casing 4, and the upper surface of sheets 1 is also made negative, and the top sheet of a stack of sheets 1 placed on the sheet receptacle 2 is sucked to the lower surface of the vacuum plate 3, and under this condition, the sheet feed roller 7 is rotated, so that the top sheet 1 is transported in the downstream direction of the sheet transport path. The sheet 1 transported from the sheet receptacle 2 by the sheet feed roller 7 is prevented from being double fed, by the sheet separation roller 8 and the frictional sheet separation plate 9 which is in elastic contact with the peripheral surface of the sheet separation roller 8, so that only one sheet is fed into the copy machine or the facsimile apparatus.
Referring to FIG. 2, in this sheet feeding apparatus, when a leding edge portion 1a of the sheet 1 fed goes beyond an end portion 3b of the vacuum plate 3 and is positioned in between the end portion 3b and the contact point of the sheet separation roller 8 between the frictional sheet separating plate 9, the leading edge portion 1a is not guided forcibly or is in the so-called guide free state. Therefore, when sheets are thin, flexible or curled, the leading edge portion 1a is not stiff enough and so long as the sheet is in the above-mentioned state, the leading edge portion 1a is prevented from being advanced by its friction with the frictional sheet separating plate 9, although the leading edge portion 1a may reach the frictional sheet separating plate 9. Therefore, the sheet waves, and adequate sheet transportation becomes difficult. The phenomenon for the leading edge portion 1a to be prevented from being advanced by its friction with the frictional sheet separating plate 9 will occur in the following cases: too small to overcome the frictional force caused by the contact of the leading edge 1a with the frictional separating plate 9, and a second case is that the leading edge 1a comes to contact with the frictional sheet separating plate 9 with an obtuse angle due to the curl of the sheet as shown by reference symbol 1A in FIG. 2. This phenomenon can be avoided to some extent if sheets with appropriate quality and thickness are selected. However, when the form, size and thickness of sheets (originals) to be fed vary as in the case of facsimile apparatus, the above-mentioned phenomenon is unavoidable in the conventional sheet feeding apparatus.